Global Change Biology (2012), doi: 10.1111/j.1365-2486.2012.02740.x
Expanded spatial extent of the Medieval Climate Anomaly revealed in lake-sediment records across the boreal region in northwest Ontario KATHLEEN R. LAIRD*, HEATHER A. HAIG*, SUSAN MA*, MELANIE V. KINGSBURY*, T H O M A S A . B R O W N † , C . F . M I C H A E L L E W I S ‡ , R O B E R T J . O G L E S B Y § and B R I A N F . CUMMING* *Paleoecological Environmental Assessment and Research Laboratory (PEARL), Department of Biology, Queen’s University, Biosciences Complex, Kingston, ON K7L 3N6, Canada, †Center for Accelerator Mass Spectrometry, Lawrence Livermore National Lab, Livermore, CA 94551, USA, ‡Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, Dartmouth, NS B2Y 4A2, Canada, §Department of Earth and Atmospheric Sciences, University of NebraskaLincoln, Lincoln, NE 68588, USA
Abstract Multi-decadal to centennial-scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within-lake calibration models developed using diatom assemblages collected from surface sediments across a water-depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near-shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate-driven influences, and can provide a sensitive record of past drought. Our lake-sediment records indicate two periods of synchronous signals, suggesting a common large-scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900-1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo-lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models. Keywords: boreal lakes, diatoms, lake level, Medieval Climate Anomaly, northwest Ontario Received 15 February 2012; revised version received 20 April 2012 and accepted 24 April 2012
Introduction Future extreme droughts, similar to or more extreme than the ‘dust-bowl’ 1930s, could be the most pressing problem of global warming (Romm, 2011). Droughts of unusually long duration or megadroughts (Cook et al., 2010b), may be an omen for the future (Seager et al., 2007; Romm, 2011), but uncertainty about their spatial extent and severity remains. Mounting evidence suggests that the Medieval Climate Anomaly (MCA, c. 900-1400 CE, common era) has a global signature (Seager et al., 2007; Trouet Correspondence: Kathleen R. Laird, tel. 613-533-6159, fax 613-5336617, e-mail: [email protected]
© 2012 Blackwell Publishing Ltd
et al., 2009; Graham et al., 2011). Regionally, however, the spatial extent, duration, and magnitude of the MCA are not clearly defined. Understanding differences in the spatial extent and severity of the MCA between the arid west and the more humid east of North America (Cook et al., 2010b) will lead to a better understanding of the MCA. Climate models suggest that a re-organization of the climate system occurred during the MCA, likely driven by changes in ocean circulation and sea-surface temperatures in both the Pacific and Atlantic Oceans (Trouet et al., 2009; Feng et al., 2011; Graham et al., 2011; Oglesby et al., 2012). Although the MCA cannot be uniformly characterized, it has distinct regional expressions (Graham et al., 2011) that are commonly manifested 1
2 K. R. LAIRD et al. as a period of prolonged drought or megadrought (Helama et al., 2009; Trouet et al., 2009; Cook et al., 2010b). In North America, the MCA was characterized by elevated aridity in the western USA (Cook et al., 2010b), and mobilized eolian dunes in the Great Plains (Miao et al., 2007). The spatial patterns of MCA droughts in North America are thought to be similar to droughts witnessed in the 1930s; however, the MCA droughts lasted for several decades to centuries thus dwarfing modern-day droughts (Seager et al., 2007; Cook et al., 2010b). Many studies suggest that these North American megadroughts were linked to persistent La Nin˜a – like conditions in the Pacific Ocean and to a positive (warm) mode of the Atlantic Multidecadal Oscillation/North Atlantic Oscillation circulation pattern (Helama et al., 2009; Trouet et al., 2009; Cook et al., 2010b; Feng et al., 2011; Oglesby et al., 2012). Lake-sediment records have been widely used to infer past climatic conditions, but regional networks of proxy records from lakes are rare. Analysis of multiple lakes within a region is the best way to distinguish changes driven by climate forcing from intrinsic local influences (Fritz, 2008; Williams et al., 2011). Water levels in many lakes are driven by changes in temperature and precipitation, and therefore can be important integrators of water balance and sentinels of water scarcity (Williamson et al., 2009). Changes in precipitation can significantly impact the hydrological system, resulting in altered patterns of runoff and concurrent change in the chemistry of lakes, which in turn may affect the physical and biological processes, such as thermal structure, light transparency, algal productivity, and community structure (Fee et al., 1996; Snucins & Gunn, 2000; Keller, 2007; Heino et al., 2009; Karlsson et al., 2009). Canada’s boreal region contains ~80–90% of the country’s freshwater (Schindler & Lee, 2010), and is predicted to be one of the biomes most affected by climate warming (Ruckstuhl et al., 2008; Schindler & Lee, 2010). One of the predictions of increasing temperatures is decreased lake levels and river flows (Schindler & Lee, 2010). Analysis of longer-term records of past water levels can provide a context for informing water managers on the inherent natural variability of lake levels and their sensitivity to climate change. Coherent shifts in past water levels of multiple lakes within a region would suggest climate as a dominant driver of change. In the Experimental Lakes Area (ELA), located in the Winnipeg River Drainage Basin (WRDB) in the boreal region of northwest Ontario, reference lakes were shown to be sensitive to the droughts of the 1980s (Schindler et al., 1996). In addition, limited long-term trends of drought from proxy records are available
from the boreal region of northwest Ontario (Laird & Cumming, 2008). Analysis of a pollen dataset across the boreal region of Canada suggests that there is a warming signal in central Canada during the MCA (Viau & Gajewski, 2009), although only two sites are close to the WRDB. Analysis of tree-rings within the WRDB over the past ~200 years suggests that low tree growth can be associated with warm/dry summers, but the signal is weak due to the generally cool, moist climate (St. George et al., 2008). This study provides the first substantive evidence of decreased water availability during the MCA across a large region of the Winnipeg River Drainage Basin (WRDB) in northwest Ontario (Fig. 1), providing convincing evidence of how far north and east the MCA extended. Furthermore, this is the first regional application of the approach proposed by Laird & Cumming (2008) to reconstruct drought from lake-sediment records in drainage lakes (open lakes with a surface outlet). This approach utilizes a sensitive relationship between changes in lake-water depth and changes in diatom assemblages derived from near-shore cores collected near the present-day ecological boundary between the shallower benthic and deeper planktonic (B : P) diatoms. A greater contribution of benthic diatoms implies shallower lake depths, and a larger contribution of planktonic diatoms infers deeper lake depths. Placement of cores near the B : P boundary can provide sensitive drought records over several millennia, and the method has the potential to be applied to numerous drainage lakes (Laird et al., 2011). In contrast, analyses of sediment records from deeper centrally located cores from drainage lakes often do not yield clear signals of changes in water level and climate (Battarbee, 2000). Here we present six diatom-inferred drought records analyzed at ~ decadal-scale resolution over the past two millennia to provide a more comprehensive view of natural variability in water resources and the degree to which climate was an important driver of the patterns.
Materials and methods
Study region The six study lakes are in the WRDB in northwest Ontario, forming a ~250 km transect across the region (Fig. 1). The lakes to the west are within the Lake of the Woods/Rainy River watershed, and those to the east in the English River watershed. All lakes are unregulated with surface outlets (open drainage lakes). Five are 1st-order headwater lakes, whereas ELA Lake 442 is a 2nd-order lake (one small pond drains into it). The lakes range in surface area from ~17 to 78 ha, and maximum lake depth from ~16–20 m (Table S1;
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
MEDIEVAL CLIMATE ANOMALY NORTHWEST ONTARIO 3
Fig. 1 Location of the study lakes (black dots) and climate stations (black stars) within the Winnipeg River Drainage Basin (WRDB). Sub-basins within the WRDB are outlined. Study sites are numbered: 1) Meekin Lake, 2) Dixie Lake, 3) ELA Lake 442, 4) ELA Lake 239, 5) Little Raleigh Lake and 6) Gall Lake.
Laird et al., 2011), with the exception of ELA Lake 239 which is 30 m deep (Laird & Cumming, 2008). Four lakes (Dixie, ELA Lake 239, ELA Lake 442 and Little Raleigh) are oligotrophic (low nutrients), with two lakes (Meekin and Gall) on the lower end of mesotrophic (moderate nutrients) (Table S1; Laird et al., 2011; Kingsbury et al., 2012). Average annual precipitation in the WRDB varies from ~750 mm in the west to ~820 mm in the east (Fig. 2a), and mean annual (and summer temperature) varying from ~2.6 (18.0) °C to ~1.5 (17.1) °C (Fig. 2b). Although average annual and summer temperatures are similar in the prairies to the west (~2.3 (18.3) °C in Winnipeg), average annual precipitation is much lower (~590 mm in Winnipeg). Our study area is predominantly covered with forests (over 70%), and only has 0.5–3% of the total land-use listed as settlement/development, with the remainder in lakes and wetlands (White, 2000). The eastern part of the WRDB consists of boreal forest comprising black spruce (Picea mariana), jack pine (Pinus banksiana), and poplar (Populus spp.), along with white birch (Betula papyrifera), balsam fir (Abies balsamea), and larch (Larix spp.) (Ontario Ministry of Natural Resources, 2000). Gall Lake, Little Raleigh Lake, and ELA lakes 239 and 442 are all located in this boreal forest zone (Kingsbury et al., 2012). Dixie Lake lies within a transitional area between the east and west vegetation habitats and its dominant species are poplar, white oak (Quercus alba), black spruce, balsam fir, jack pine, and white spruce (Picea glauca). Meekin Lake is located in the western-most vegetation zone which comprises black ash (Fraxinus nigra), poplar, white birch, red maple (Acer rubrum), and balsam fir (Kingsbury et al., 2012).
Core location The locations of the gravity and piston cores were chosen on the basis of the results from seismic profiling of the sediment that identified regions of gentle lake-bottom slope with sufficient sediment accumulation for decadal analysis over the past several thousand years. Seismic profiling was carried out using a Knudsen (Perth, Ontario, Canada) 320M marine sounder with 200 kHz and 28 kHz transducers. Lake floor and sub-bottom acoustic reflections were recorded graphically on thermal recording media. The cores were retrieved (Glew et al., 2001) ~1–2 m deeper than the depth of the present-day B : P diatom boundary (Fig. 3), as defined from a surface-sediment calibration dataset from within each lake (Laird et al., 2011). Data on the specific diatom assemblages that define the benthic and planktonic zones in the surface samples can be found in Kingsbury et al. (2012). Analysis of multiple transects from three different sub-basins of a large boreal lake in the WRDB with varying wind exposure and bottom slopes, indicated that a single, well-placed transect may be used to define this B : P diatom boundary within a lake (Laird et al., 2010). Cores were sectioned into 0.25 cm intervals, gravity cores in the field (Glew et al., 2001) and piston cores in the lab.
Laboratory procedures For each core, ~0.2–0.3 g of wet sediment was sub-sampled and placed in 20 mL glass vials to which a 1 : 1 mixture by molar weight of concentrated nitric (HNO3) and sulfuric (H2SO4) acid was used to remove organic matter. The sam-
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
4 K. R. LAIRD et al.
Fig. 2 Precipitation (a) from 1915 to 2009 and temperature records (b) from 1915 to 2010 for Winnipeg, Manitoba, and climate stations closest to our study sites in Ontario: Kenora, Dryden, and Sioux Lookout. The dashed line indicates the mean of the record. The solid line indicates the linear trend, with P-values indicated for those records with significant trends. The 1930s, 1950s, and 1980s are highlighted for ease of comparison. All data are from the Adjusted and Homogenized Canadian Climate Data archive (http://ec.gc.ca/ dccha-ahccd). The Winnipeg and Dryden precipitation records from this dataset only extended to 2006 and 2003, respectively.
Fig. 3 Schematic of the sensitivity of core location for tracking the movement of lake depth associated with the benthic:planktonic (B : P) diatom boundary under different climate scenarios: (a) wetter conditions, precipitation (P) > evaporation (E); and (b) more arid conditions, P < E. The solid lines indicate the present-day B : P boundary, the dotted lines indicate the direction of movement of the B : P boundary. Under climate scenario (a) as lake level rises, the B : P boundary moves toward shore. Under climate scenario (b) as lake level declines, the B : P boundary moves toward the deeper center areas. Figure is modified from Laird et al., 2011.
ples were allowed to settle for 24 h before the acid above the sample was removed, and the sample was rinsed with distilled water. This procedure was repeated until the sample had the same pH as the distilled water (approximately eight rinses). Four successive dilutions for each sample were pipetted onto coverslips ensuring that each sample was well mixed.
Samples on the coverslips were air-dried overnight, then heated on a warming plate to remove any remaining moisture, and subsequently mounted with Naphrax® onto glass microscope slides. Diatoms were identified and counted along transects on the prepared slide using a Leica (DMRB model) microscope fitted with a 100x fluotar objective
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
MEDIEVAL CLIMATE ANOMALY NORTHWEST ONTARIO 5 (NA = 1.3) and using differential interference contrast optics at 1000x magnification. Approximately 400 diatom valves were enumerated per slide. Diatoms were identified down to the species level or lower, using taxonomic references outlined in Laird et al. (2011).
Statistical models Diatom-inferred (D-I) depth models were developed based on surface-sediment samples (~top 0.5 cm interval) collected along a depth gradient within each of the six small boreal lakes (Laird & Cumming, 2008; Laird et al., 2011). A series of surface-sediment samples were collected along a gentle slope ( ~15 m (Laird & Cumming, 2008, 2009), and the complexities of the potential for ELA Lake 239 coalescing with adjacent ELA Lake 240 with increasing depth. A widespread external forcing must be large enough for regional patterns to emerge, despite the potential heterogeneity and intrinsic complexities of local ecosystems and internal dynamics (Williams et al., 2011). In the Nebraska Sandhills (one of the few other regions with a paleolimnologicaldrought network), an analysis of five topographically closed lakes indicated relative coherency over the last 4000 years, particularly during the MCA with all lakes indicating lake-level decline (Schmieder et al., 2011). Our sites in the WRDB indicate some temporal variability, with an earlier onset in the west (~900 CE) vs. somewhat later in the east (~1000–1100 CE), indicating a potential time-transgressive response, although these periods nearly overlap when errors in radiocarbon are taken into account. Our data indicates the MCA penetrated at least as far as our easternmost lake, but without sites further east we cannot clearly define the eastward extent. In the Canadian and northern U.S. prairies to the west of the WRDB, there is high variability in diatominferred effective moisture during the MCA interval (Laird et al., 2003). This may be due to complex interactions between groundwater and climate in this region, as well as differences in the spatial pattern of precipitation and drought depending on the position and configuration of the jet stream and high-pressure ridges (Laird et al., 2003). Other proxy records from regions adjacent to the WRDB suggest intervals of drought within the MCA interval. In Minnesota, sand deposits in Mina Lake indicate large declines in lake level during the 1300s (St. Jacques et al., 2008), high eolian deposition occurred from ~1280 to 1410 CE in Elk Lake (Dean, 1997) and d18O from calcite indicated an arid period from ~1100 to 1400 CE in Steel Lake (Tian et al., 2006). In Manitoba, the cellulose d18O record from the southern basin of Lake Winnipeg indicated severe dry conditions between 1180 and 1230 CE, and a less-severe dry period from 1320 to 1340 CE (Buhay et al., 2009). Relative warm conditions during the MCA in comparison to the Little Ice Age (LIA) have been inferred from pollen records in the central boreal region of Canada and in Wisconsin
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
10 K . R . L A I R D e t a l . (Viau & Gajewski, 2009; Viau et al., 2012; Wahl et al., 2012). Many studies now suggest that the MCA was of global extent, with many regions indicating dry conditions, while others were wet (Seager et al., 2007; Helama et al., 2009; Trouet et al., 2009; Graham et al., 2011). These conclusions are based on proxy records around the world and climate models that suggest a re-organization of the climate system. While many of these studies point to conditions in the Pacific Ocean being in a persistent La Nin˜a mode, others also indicate that conditions in the North Atlantic (Feng et al., 2011; Oglesby et al., 2012), and Indian Ocean (Graham et al., 2011) also played a role in this re-organization. Observational and modeling studies also suggest that these same sea-surface temperature (SST) patterns play a major role in present-day droughts across central North America, including our study region (e.g., Hu et al., 2011). It is not that an entirely different set of controls accounted for the drought; rather the SST patterns favoring drought appear to have prevailed during much of the MCA, whereas they are much more sporadic at present.
Recent signal (last ~100 years) Recent studies show that anthropogenic forcing has a detectable influence on climate, being implicated in precipitation trends across large latitudinal bands (Zhang et al., 2007), as well as trends in river flow, winter air temperature and snow pack (Barnett et al., 2008). At the ELA, a significant upward trend in average annual temperature (mostly due to temperature increases in the winter) is accompanied by increasing rainfall, particularly between April to October (Parker et al., 2009). These recent increases in precipitation in our study region (Fig. 2a) may be related to anticipated increased poleward moisture transport into the Great Lakes region related to greenhouse-gas induced warming (Kutzbach et al., 2005). An increase in precipitation is certainly a factor in the most recent increases in the inferred depth of our lakes. However, warming air temperatures can also be associated with a warmer epilimnion in lakes which can lead to increased stratification (Schindler et al., 1996), conditions that would favor the small planktonic Discostella (formerly Cyclotella) taxa (Ru¨hland et al., 2008), the dominant planktonic diatoms in all of our lakes (Fig. S1, Table S3). In three of our lakes (Meekin, ELA L442, Gall), the percentages of Discostella taxa are at their highest during the last 100 years of the 2000-year records. Thus the magnitude of the inferred highs of these lakes was likely amplified, in part, by increased stratification. Nonetheless, this does not negate the fact that all lakes show a similar
increasing trend in inferred depth over the last 100 years, even though the degree of change may be a result from complex interaction of climatic influences on the physical properties of lakes. In Meekin Lake, there were also concurrent small increases (~2%) in two other planktonic taxa, Asterionella formosa and Fragilaria nanana (Laird et al., 2011), which may imply either increased nutrients in the metalimnion (Ru¨hland et al., 2010) or increased stratification (Longhi & Beisner, 2009). Although nutrients could potentially be a confounding influence on our inferred water-depth changes, we found that even in our moderate nutrient status lakes (Meekin and Gall, ~10–12 lg L 1 total phosphorus, TP) the assemblages were dominated by oligotrophic taxa (including planktonic Discostella), with only small percentages of more mesotrophic taxa, and the assemblages had a similar distribution with changes in lake depth in all other lakes studied (Kingsbury et al., 2012). Gall Lake has the highest TP levels (~12 lg L 1) of our study lakes, but the percent abundance of more mesotrophic taxa, such as A. formosa has changed little over the past 2000 years and never exceeded 10% of the assemblage (Haig, 2011), suggesting nutrient status in this lake has not changed substantially over this time period. Potential multiple stressors on lakes from anthropogenic influences again highlights the necessity of a regional network of sites to help decipher intrinsic factors from external forcings.
Application of this approach to other regions Regional networks of proxy-lake records provide a means of distinguishing climate-driven signals from more local influences. Our approach of utilizing nearshore sediments, as a sensitive method to track changes in diatom assemblages near the benthic to planktonic (B : P) boundary, led to the documentation of an arid MCA across the WRDB in northwest Ontario, providing convincing evidence on the northward spatial extent of this megadrought. The ability to develop robust diatom-inferred depth models across many lakes and a well-defined B : P boundary, makes this approach potentially applicable to regions dominated by drainage lakes. Consequently, this approach can provide a method to estimate changes in effective moisture in comparison to conditions today, in more humid regions where such data have been extremely difficult to acquire. Tracking changes in diatom assemblages near the B : P diatom boundary in lake-sediment cores provides a means of retrieving a drought signal from dilute lakes with outlets; however, this approach is most appropriate in small lakes (< 100–200 ha) with a gently sloping bottom that has sufficient near-shore sediment accumu-
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
M E D I E V A L C L I M A T E A N O M A L Y N O R T H W E S T O N T A R I O 11 lation. The location of the core near the present-day depth of the B : P diatom boundary (Fig. 3) makes this technique especially sensitive to detecting even lowintensity droughts (Laird et al., 2011), but needs to be based on a high-resolution (~1 m) calibration of diatom assemblages in surface sediments across a water-depth gradient in each study lake (Kingsbury et al., 2012). Although lower sedimentation rates are generally associated with near-shore cores, comparison across lakes is still realistic at multi-decadal to centennial scales. Given these caveats, this approach can provide a powerful means to reconstruct climate where more traditional paleolimnological methods based on a central core from drainage lakes are inadequate (Battarbee, 2000).
Future implications Empirical evidence for the penetration of the MCA drought into northwest Ontario, fills a geographical gap that climate models can strive to mimic for a better mechanistic understanding of the spatial pattern of the MCA. The models must now be able to simulate this expanded region of drought if they are to be considered useful in understanding how the MCA came to be. This is especially important for high spatial resolution (4–12 km), regional climate models. Current increasing trends in precipitation in northwest Ontario suggest that water scarcity may not be an issue in the near future. However, if the concurrent increasing trends in temperature continue, this could result in declines of overall effective moisture. If trends in effective moisture were to decrease, this could lead to the return of aridity, resulting in significant declines in hydropower generation, and other environmental and socioeconomic impacts. Expanding our knowledge of the spatial extent and severity of past megadroughts provides plausible scenarios for water-resource management and societal impacts. Past megadroughts have been implicated in the decline, abandonment, and collapse of societies. The collapse of the classic Maya in Mexico and the Tiwanaku in the Bolivian-Peruvian altiplano was coincident with prolonged drought (de Menocal, 2001). Large population declines and abandonment of Puebloan and Cahokia settlements in North America occurred during the megadroughts of the MCA (Cook et al., 2007). Recent tree-ring records in Asia suggest that drought may also have contributed to the fall of the Ming Dynasty (Cook et al., 2010a). What the future climate may hold for regions of North America due to global warming is unclear, but a return to the aridity of the MCA would have significant ecological, environmental, and social impacts on this region.
Acknowledgements We thank Claire Broughton, Lindsay Braeger, Thane Anderson, and Brendan Wiltse for assistance with field work. Marten Douma of the Geological Survey of Canada provided equipment and training for acquisition of lake-sediment seismic profiles. D. L. Forbes and C.T. Schafer reviewed an early draft of the paper for the GSC. Water chemistry was provided by Andrew Paterson at the Ontario Ministry of Environment, Dorset Environmental Center. We thank the three anonymous reviewers for their comments which strengthened this paper. Funding for this project comes from an NSERC Collaborative Research and Development grant in partnership with Manitoba Hydro. Conflict of interest: No authors have a conflict of interest. Author contributions K.L. and B.C. designed the study and wrote the grant proposal and paper; K.L., H.H., S.M., M.K. provided data and analyses; T.B. provided the carbon dating; M.L. provided seismic profile data; R.O. provided input of synoptic climatic interpretations. All authors contributed to the discussion, interpretation, and writing of the paper.
References Barnett TP, Pierce DW, Hidalgo HG et al. (2008) Human-induced changes in the hydrology of the Western United States. Science, 319, 1080–1083. Battarbee RW (2000) Palaeolimnological approaches to climate change, with special regard to the biological record. Quaternary Science Reviews, 19, 107–124. Bennett KD (1996) Determination of the number of zones in a biostratigraphical sequence. New Phytology, 132, 155–170. Buhay WM, Simpson S, Thorleifson H et al. (2009) A 1000 year record of dry conditions in the eastern Canadian prairies reconstructed from oxygen and carbon isotope measurements on Lake Winnipeg sediment organics. Journal of Quaternary Science, 24, 426–436. Cook ER, Seager R, Cane MA, Stahle DW (2007) North American drought: reconstructions, causes, and consequences. Earth Science Reviews, 81, 93–134. Cook ER, Anchukaitis KJ, Buckley BM, D’Arrigo RD, Jacoby GC, Wright WE (2010a) Asian monsoon failure and megadrought during the last millennium. Science, 328, 486–489. Cook ER, Seager R, Heim RR Jr., Vose RS, Herweijer C, Woodhouse C (2010b) Megadroughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science, 25, 48–61. Dean WE (1974) Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. Journal of Sedimentology Petrology, 44, 242–248. Dean WE (1997) Rates, timing, and cyclicity of Holocene eolian activity in north-central US: evidence from varved lake sediments. Geology, 25, 331–334. Dearing JA (1997) Sedimentary indicators of lake-level changes in the humid temperate zone: a critical review. Journal of Paleolimnology, 18, 1–14. Fee EJ, Hecky RE, Kasian SEM, Cruikshank DR (1996) Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian shield lakes. Limnology and Oceanography, 41, 912–920. Feng S, Hu Q, Oglesby RJ (2011) Influence of Atlantic sea surface temperature on persistent drought in North America. Climate Dynamics, 37, 569–586. Findlay DL, Kasian SEM, Stainton MP, Beaty K, Lyng M (2001) Climatic influences on algal populations of boreal forest lakes in the Experimental Lakes Area. Limnology and Oceanography, 46, 1784–1793. Fritz SC (2008) Deciphering climatic history from lake sediments. Journal of Paleolimnology, 39, 5–16. Glew JR (1989) A new trigger mechanism for sediment samplers. Journal of Paleolimnology, 2, 241–243. Glew JR, Smol JP, Last WM (2001) Sediment core collection and extrusion. In Tracking Environmental Change Using Lake Sediments: Volume 1. Basin Analysis, Coring, and Chronological Techniques (eds. Last WM, Smol JP), pp. 73–105. Kluwer Academic Publishers, Dordrecht. Graham NE, Ammann CM, Fleitmann D, Cobb KM, Luterbacher J (2011) Support for global climate reorganization during the ‘Medieval Climate Anomaly’. Climate Dynamics, 37, 1217–1245. Haig H (2011) Diatom-inferred Changes in Effective Moisture from Gall Lake, Northwestern Ontario, Over the Past Two Millennia. M.Sc. thesis, Queen’s University, Kingston, Ontario.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
12 K . R . L A I R D e t a l . Heino J, Virkkala R, Toivonen H (2009) Climate change and freshwater biodiversity: detected patterns, future trends and adaptations in northern regions. Biological
Experimental Lakes Area (Ontario, Canada). Canandian Journal of Fisheries and Aquatic Sciences, 66, 1848–1863.
Reviews, 84, 39–54. Helama S, Merila¨inen J, Tuomenvirta H (2009) Multicentennial megadrought in northern Europe coincided with a global El nin˜o -southern oscillation drought pattern during the Medieval Climate Anomaly. Geology, 37, 175–178. Hu Q, Feng S, Oglesby RJ (2011) Variations in north American summer precipitation driven by the Atlantic multidecadal oscillation. Journal of Climate, 24, 5555–5570. Juggins S (2003) C2 Software for Ecological and Palaeoecological Data Analysis and Visuali-
Reimer PJ, Baillie MGL, Bard E et al. (2009) INTCAL09 and MARINE09 radiocarbon age calibration curves, 0-50 000 years cal BP. Radiocarbon, 51, 1111–1150. Romm J (2011) The next dust bowl. Nature, 478, 450–451. Ruckstuhl KE, Johnson EA, Myianishi K (2008) The boreal forest and global change. Philosophical Transactions of the Royal Society B, 363(1501), 2245–2249. Ru¨hland K, Paterson AM, Smol JP (2008) Hemispheric-scale patterns of climaterelated shifts in planktonic diatoms from North American and European lakes.
zation User Guide Version 1.3. University of Newcastle, Newcastle, UK. Karlsson J, Bystro P, Ask J, Persson L, Jansson M (2009) Light limitation of nutrientpoor lake ecosystems. Nature, 460, 506–510. Keller W (2007) Implications of climate warming for Boreal shield lakes: a review and synthesis. Environmental Reviews, 15, 99–112. Kingsbury MV, Laird KR, Cumming BF (2012) Consistent patterns in diatom
Global Change Biology, 14, 2740–2754. Ru¨hland KM, Paterson AM, Hargan K, Jenkin A, Clark BJ, Smol JP (2010) Reorganization of algal communities in the lake of the Woods (Ontario, Canada) in response to turn-of-the-century damming and recent warming. Limnology and Oceanography, 55, 2433–2451. Schelske CL, Peplow A, Brenner M, Spencer CN (1994) Low-background gamma
assemblages and diversity measures across water-depth gradients from eight Boreal lakes from northwestern Ontario (Canada). Freshwater Biology, 57, 1151– 1165. Kutzbach JE, Williams JW, Vavrus SJ (2005) Simulated 21st century changes in regional water balance of the Great lakes region and links to changes in global temperature and poleward moisture transport. Geophysical Research Letters, 32, L17707, doi:10.1029/2005GL023506.
counting: applications for 210Pb dating of sediments. Journal of Paleolimnology, 10, 115–128. Schindler DW, Donahue WF (2006) An impending water crisis in Canada’s western prairie provinces. Proceedings of the National Academy of Sciences, 103, 7210–7216. Schindler DW, Lee PG (2010) Comprehensive conservation planning to protect biodiversity and ecosystem services in Canadian boreal regions under a warming climate and increasing exploitation. Biological Conservation, 143, 1571–1586.
Laird KR, Cumming BF (2008) Reconstruction of Holocene lake level from diatoms, chrysophytes and organic matter in a drainage lake from the Experimental Lakes Area (northwestern Ontario, Canada). Quaternary Research, 69, 292–305. Laird KR, Cumming BF (2009) Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area, northwestern Ontario, Canada. Journal of Paleolimnology, 42, 65–80.
Schindler DW, Bayley SE, Parker BR et al. (1996) The effects of climatic warming on the properties of boreal lakes and streams at the Experimental Lakes Area, northwestern Ontario. Limnology and Oceanography, 41, 1004–1017. Schmieder J, Fritz SC, Swinehart JB et al. (2011) A regional-scale climate reconstruction of the last 4000 years from lakes in the Nebraska sand hills, USA. Quaternary Science Reviews, 30, 1797–1812.
Laird KR, Cumming BF, Wunsam S, Rusak JA, Oglesby RJ, Fritz SC, Leavitt PR (2003) Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia. Proceedings National Academy of Science, 100, 2483–2488. Laird KR, Kingsbury MV, Cumming BF (2010) Assessment of diatom species habitats, species diversity and depth-inference models across multiple transects in Worth lake, northwestern Ontario, Canada. Journal of Paleolimnology, 44, 1009–
Seager R, Graham N, Herweijer C, Gorodn AL, Kushnir Y, Cook E (2007) Blueprints for medieval hydroclimate. Quaternary Science Reviews, 26, 2322–2336. Snucins E, Gunn J (2000) Interannual variation in the thermal structure of clear and colored lakes. Limnology and Oceanography, 45, 1639–1646. St. George S (2007) Streamflow in the Winnipeg river basin, Canada: trends, extremes and climate linkages. Journal of Hydrology, 332, 396–411. St. George S, Meko DM, Evans MN (2008) Regional tree growth and inferred summer
1024. Laird KR, Kingsbury MV, Lewis CFM, Cumming BF (2011) Diatom-inferred depth models in 8 Canadian Boreal lakes: inferred changes in the benthic:planktonic depth boundary and implications for assessment of past droughts. Quaternary Science Reviews, 30, 1201–1217. Longhi ML, Beisner BE (2009) Environmental factors controlling the vertical distribu-
climate in the Winnipeg river basin, Canada, since AD 1783. Quaternary Research, 70, 158–172. St. Jacques JM, Cumming BF, Smol JP (2008) A 900-year pollen-inferred temperature and effective moisture record from varved lake mina, west-central Minnesota, USA. Quaternary Science Reviews, 27, 781–796. Stuiver M, Reimer PJ (1993) Extended 14C database and revised CALIB radiocarbon
tion of phytoplankton in lakes. Journal of Plankton Research, 31, 1195–1207. Ma S (2011) Climate change and water availability over the last two millennia in Little Raleigh lake, northwestern Ontario. M.Sc. thesis, Queen’s University, Kingston, Ontario. Mekis E´, Vincent LA (2011) An overview of the second generation adjusted daily precipitation dataset for trend analysis in Canada. Atmosphere-Ocean, 49, 163–177. de Menocal PB (2001) Cultural responses to climate during the late Holocene. Science,
calibration program. Radiocarbon, 35, 315–330. Tian J, Nelson DM, Hu FS (2006) Possible linkages of late-Holocene drought in the North American midcontinent to Pacific decadal oscillation and solar activity. Geophysical Research Letters, 33, L23702, doi: 10.1029/2006GL028169 Trouet V, Esper J, Graham NE, Baker A, Scourse JD, Frank DC (2009) Persistent positive North Atlantic oscillation mode dominated the Medieval Climate Anomaly. Science, 324, 78–80.
292, 667–673. Miao X, Mason JA, Swinehart JB, Loope DB, Hanson PR, Goble RJ, Xiaodong L (2007) A 10 000 year record of dune activity, dust storms, and severe drought in the central Great plains. Geology, 35, 119–122. Moos MT, Laird KR, Cumming BF (2009) Climate-related eutrophication of a small boreal lake in northwestern Ontario: a palaeolimnological perspective. The Holo-
Viau AE, Gajewski K (2009) Reconstructing millennial-scale, regional paleoclimates of boreal Canada during the Holocene. Journal of Climate, 22, 316–330. Viau AE, Ladd M, Gajewski K (2012) The climate of North America during the past 2000 years reconstructed from pollen data. Global and Planetary Change, 84-85, 7, 5–83. Wahl ER, Diaz HF, Ohlwein C (2012) A pollen-based reconstruction of summer temperature in central North America and implications for circulation patterns during
cene, 19, 359–367. Oglesby R, Feng S, Hu Q, Rowe C (2012) The role of the Atlantic multidecadal oscillation on medieval drought in North America: Synthesizing results from proxy data and climate models. Global and Planetary Change, 84-85, 56–65. Oldfield F, Appleby PG (1985) Empirical testing of 210Pb-dating models for lake sediments. In Lake Sediments and Environmental History (eds Haworth EY, Lund JWG), pp. 93–124. University of Minnesota Press, Minneapolis.
medieval times. Global and Planetary Change, 84-85, 6, 6–74. White DB (2000) Ontario Land Cover Data. G.A.D.A.S. Section & N.R.I. Branch. Ontario Ministry of Natural Resources, Peterborough, Ontario. Williams JW, Blois JL, Shuman BN (2011) Extrinsic and intrinsic forcing of abrupt ecological change: case studies from the late Quaternary. Journal of Ecology, 99, 664–677. Williamson CE, Saros JE, Schindler DW (2009) Sentinels of change: lakes and reservoirs provide key insights into the effects and mechanisms of climate change. Sci-
Ontario Ministry of Natural Resources (2000) Forest Resources Inventory (FRI) Database, Ontario Ministry of Natural Resources, Queen’s University, Kingston ON. Pace ML, Cole JJ (2002) Synchronous variation of dissolved organic carbon and color in lakes. Limnology and Oceanography, 47, 333–342. Parker BR, Schindler DW, Beaty KG, Stainton MP, Kasian SEM (2009) Long-term changes in climate, streamflow, nutrient budgets for first-order catchments at the
ence, 323, 887–888. Winter TC, Woo M (1990) Hydrology of lakes and wetlands. In Surface Water Hydrology (eds Wolman MG, Riggs HC), pp. 158–188. Geological Society of America, Boulder, CO. Zhang X, Zwiers FW, Hegerl GC et al. (2007) Detection of human influence on twentieth-century precipitation trends. Nature, 448, 461–465.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
M E D I E V A L C L I M A T E A N O M A L Y N O R T H W E S T O N T A R I O 13 Supporting Information Additional Supporting Information may be found in the online version of this article: Figure S1. Percentage of planktonic diatoms in the cores retrieved from the six study lakes. Figure S2. Percentage of benthic diatoms in the cores retrieved from the six study lakes. Table S1. Summary of some physical and chemical characteristics for each of the study lakes Table S2. Results of AMS radiocarbon dates processed at the Center for AMS at Lawrence Livermore National Laboratory Table S3. List of dominant and sub-dominant planktonic diatom taxa for each of the six lakes Table S4. List of dominant and sub-dominant benthic diatom taxa for each of the six lakes
Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
Expanded spatial extent of the Medieval Climate Anomaly revealed in lake-sediment records across the boreal region in northwest Ontario KATHLEEN R. LAIRD*, HEATHER A. HAIG*, SUSAN MA*, MELANIE V. KINGSBURY*, T H O M A S A . B R O W N † , C . F . M I C H A E L L E W I S ‡ , R O B E R T J . O G L E S B Y § and B R I A N F . CUMMING* *Paleoecological Environmental Assessment and Research Laboratory (PEARL), Department of Biology, Queen’s University, Biosciences Complex, Kingston, ON K7L 3N6, Canada, †Center for Accelerator Mass Spectrometry, Lawrence Livermore National Lab, Livermore, CA 94551, USA, ‡Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, Dartmouth, NS B2Y 4A2, Canada, §Department of Earth and Atmospheric Sciences, University of NebraskaLincoln, Lincoln, NE 68588, USA
Abstract Multi-decadal to centennial-scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within-lake calibration models developed using diatom assemblages collected from surface sediments across a water-depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near-shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate-driven influences, and can provide a sensitive record of past drought. Our lake-sediment records indicate two periods of synchronous signals, suggesting a common large-scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900-1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo-lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models. Keywords: boreal lakes, diatoms, lake level, Medieval Climate Anomaly, northwest Ontario Received 15 February 2012; revised version received 20 April 2012 and accepted 24 April 2012
Introduction Future extreme droughts, similar to or more extreme than the ‘dust-bowl’ 1930s, could be the most pressing problem of global warming (Romm, 2011). Droughts of unusually long duration or megadroughts (Cook et al., 2010b), may be an omen for the future (Seager et al., 2007; Romm, 2011), but uncertainty about their spatial extent and severity remains. Mounting evidence suggests that the Medieval Climate Anomaly (MCA, c. 900-1400 CE, common era) has a global signature (Seager et al., 2007; Trouet Correspondence: Kathleen R. Laird, tel. 613-533-6159, fax 613-5336617, e-mail: [email protected]
© 2012 Blackwell Publishing Ltd
et al., 2009; Graham et al., 2011). Regionally, however, the spatial extent, duration, and magnitude of the MCA are not clearly defined. Understanding differences in the spatial extent and severity of the MCA between the arid west and the more humid east of North America (Cook et al., 2010b) will lead to a better understanding of the MCA. Climate models suggest that a re-organization of the climate system occurred during the MCA, likely driven by changes in ocean circulation and sea-surface temperatures in both the Pacific and Atlantic Oceans (Trouet et al., 2009; Feng et al., 2011; Graham et al., 2011; Oglesby et al., 2012). Although the MCA cannot be uniformly characterized, it has distinct regional expressions (Graham et al., 2011) that are commonly manifested 1
2 K. R. LAIRD et al. as a period of prolonged drought or megadrought (Helama et al., 2009; Trouet et al., 2009; Cook et al., 2010b). In North America, the MCA was characterized by elevated aridity in the western USA (Cook et al., 2010b), and mobilized eolian dunes in the Great Plains (Miao et al., 2007). The spatial patterns of MCA droughts in North America are thought to be similar to droughts witnessed in the 1930s; however, the MCA droughts lasted for several decades to centuries thus dwarfing modern-day droughts (Seager et al., 2007; Cook et al., 2010b). Many studies suggest that these North American megadroughts were linked to persistent La Nin˜a – like conditions in the Pacific Ocean and to a positive (warm) mode of the Atlantic Multidecadal Oscillation/North Atlantic Oscillation circulation pattern (Helama et al., 2009; Trouet et al., 2009; Cook et al., 2010b; Feng et al., 2011; Oglesby et al., 2012). Lake-sediment records have been widely used to infer past climatic conditions, but regional networks of proxy records from lakes are rare. Analysis of multiple lakes within a region is the best way to distinguish changes driven by climate forcing from intrinsic local influences (Fritz, 2008; Williams et al., 2011). Water levels in many lakes are driven by changes in temperature and precipitation, and therefore can be important integrators of water balance and sentinels of water scarcity (Williamson et al., 2009). Changes in precipitation can significantly impact the hydrological system, resulting in altered patterns of runoff and concurrent change in the chemistry of lakes, which in turn may affect the physical and biological processes, such as thermal structure, light transparency, algal productivity, and community structure (Fee et al., 1996; Snucins & Gunn, 2000; Keller, 2007; Heino et al., 2009; Karlsson et al., 2009). Canada’s boreal region contains ~80–90% of the country’s freshwater (Schindler & Lee, 2010), and is predicted to be one of the biomes most affected by climate warming (Ruckstuhl et al., 2008; Schindler & Lee, 2010). One of the predictions of increasing temperatures is decreased lake levels and river flows (Schindler & Lee, 2010). Analysis of longer-term records of past water levels can provide a context for informing water managers on the inherent natural variability of lake levels and their sensitivity to climate change. Coherent shifts in past water levels of multiple lakes within a region would suggest climate as a dominant driver of change. In the Experimental Lakes Area (ELA), located in the Winnipeg River Drainage Basin (WRDB) in the boreal region of northwest Ontario, reference lakes were shown to be sensitive to the droughts of the 1980s (Schindler et al., 1996). In addition, limited long-term trends of drought from proxy records are available
from the boreal region of northwest Ontario (Laird & Cumming, 2008). Analysis of a pollen dataset across the boreal region of Canada suggests that there is a warming signal in central Canada during the MCA (Viau & Gajewski, 2009), although only two sites are close to the WRDB. Analysis of tree-rings within the WRDB over the past ~200 years suggests that low tree growth can be associated with warm/dry summers, but the signal is weak due to the generally cool, moist climate (St. George et al., 2008). This study provides the first substantive evidence of decreased water availability during the MCA across a large region of the Winnipeg River Drainage Basin (WRDB) in northwest Ontario (Fig. 1), providing convincing evidence of how far north and east the MCA extended. Furthermore, this is the first regional application of the approach proposed by Laird & Cumming (2008) to reconstruct drought from lake-sediment records in drainage lakes (open lakes with a surface outlet). This approach utilizes a sensitive relationship between changes in lake-water depth and changes in diatom assemblages derived from near-shore cores collected near the present-day ecological boundary between the shallower benthic and deeper planktonic (B : P) diatoms. A greater contribution of benthic diatoms implies shallower lake depths, and a larger contribution of planktonic diatoms infers deeper lake depths. Placement of cores near the B : P boundary can provide sensitive drought records over several millennia, and the method has the potential to be applied to numerous drainage lakes (Laird et al., 2011). In contrast, analyses of sediment records from deeper centrally located cores from drainage lakes often do not yield clear signals of changes in water level and climate (Battarbee, 2000). Here we present six diatom-inferred drought records analyzed at ~ decadal-scale resolution over the past two millennia to provide a more comprehensive view of natural variability in water resources and the degree to which climate was an important driver of the patterns.
Materials and methods
Study region The six study lakes are in the WRDB in northwest Ontario, forming a ~250 km transect across the region (Fig. 1). The lakes to the west are within the Lake of the Woods/Rainy River watershed, and those to the east in the English River watershed. All lakes are unregulated with surface outlets (open drainage lakes). Five are 1st-order headwater lakes, whereas ELA Lake 442 is a 2nd-order lake (one small pond drains into it). The lakes range in surface area from ~17 to 78 ha, and maximum lake depth from ~16–20 m (Table S1;
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
MEDIEVAL CLIMATE ANOMALY NORTHWEST ONTARIO 3
Fig. 1 Location of the study lakes (black dots) and climate stations (black stars) within the Winnipeg River Drainage Basin (WRDB). Sub-basins within the WRDB are outlined. Study sites are numbered: 1) Meekin Lake, 2) Dixie Lake, 3) ELA Lake 442, 4) ELA Lake 239, 5) Little Raleigh Lake and 6) Gall Lake.
Laird et al., 2011), with the exception of ELA Lake 239 which is 30 m deep (Laird & Cumming, 2008). Four lakes (Dixie, ELA Lake 239, ELA Lake 442 and Little Raleigh) are oligotrophic (low nutrients), with two lakes (Meekin and Gall) on the lower end of mesotrophic (moderate nutrients) (Table S1; Laird et al., 2011; Kingsbury et al., 2012). Average annual precipitation in the WRDB varies from ~750 mm in the west to ~820 mm in the east (Fig. 2a), and mean annual (and summer temperature) varying from ~2.6 (18.0) °C to ~1.5 (17.1) °C (Fig. 2b). Although average annual and summer temperatures are similar in the prairies to the west (~2.3 (18.3) °C in Winnipeg), average annual precipitation is much lower (~590 mm in Winnipeg). Our study area is predominantly covered with forests (over 70%), and only has 0.5–3% of the total land-use listed as settlement/development, with the remainder in lakes and wetlands (White, 2000). The eastern part of the WRDB consists of boreal forest comprising black spruce (Picea mariana), jack pine (Pinus banksiana), and poplar (Populus spp.), along with white birch (Betula papyrifera), balsam fir (Abies balsamea), and larch (Larix spp.) (Ontario Ministry of Natural Resources, 2000). Gall Lake, Little Raleigh Lake, and ELA lakes 239 and 442 are all located in this boreal forest zone (Kingsbury et al., 2012). Dixie Lake lies within a transitional area between the east and west vegetation habitats and its dominant species are poplar, white oak (Quercus alba), black spruce, balsam fir, jack pine, and white spruce (Picea glauca). Meekin Lake is located in the western-most vegetation zone which comprises black ash (Fraxinus nigra), poplar, white birch, red maple (Acer rubrum), and balsam fir (Kingsbury et al., 2012).
Core location The locations of the gravity and piston cores were chosen on the basis of the results from seismic profiling of the sediment that identified regions of gentle lake-bottom slope with sufficient sediment accumulation for decadal analysis over the past several thousand years. Seismic profiling was carried out using a Knudsen (Perth, Ontario, Canada) 320M marine sounder with 200 kHz and 28 kHz transducers. Lake floor and sub-bottom acoustic reflections were recorded graphically on thermal recording media. The cores were retrieved (Glew et al., 2001) ~1–2 m deeper than the depth of the present-day B : P diatom boundary (Fig. 3), as defined from a surface-sediment calibration dataset from within each lake (Laird et al., 2011). Data on the specific diatom assemblages that define the benthic and planktonic zones in the surface samples can be found in Kingsbury et al. (2012). Analysis of multiple transects from three different sub-basins of a large boreal lake in the WRDB with varying wind exposure and bottom slopes, indicated that a single, well-placed transect may be used to define this B : P diatom boundary within a lake (Laird et al., 2010). Cores were sectioned into 0.25 cm intervals, gravity cores in the field (Glew et al., 2001) and piston cores in the lab.
Laboratory procedures For each core, ~0.2–0.3 g of wet sediment was sub-sampled and placed in 20 mL glass vials to which a 1 : 1 mixture by molar weight of concentrated nitric (HNO3) and sulfuric (H2SO4) acid was used to remove organic matter. The sam-
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
4 K. R. LAIRD et al.
Fig. 2 Precipitation (a) from 1915 to 2009 and temperature records (b) from 1915 to 2010 for Winnipeg, Manitoba, and climate stations closest to our study sites in Ontario: Kenora, Dryden, and Sioux Lookout. The dashed line indicates the mean of the record. The solid line indicates the linear trend, with P-values indicated for those records with significant trends. The 1930s, 1950s, and 1980s are highlighted for ease of comparison. All data are from the Adjusted and Homogenized Canadian Climate Data archive (http://ec.gc.ca/ dccha-ahccd). The Winnipeg and Dryden precipitation records from this dataset only extended to 2006 and 2003, respectively.
Fig. 3 Schematic of the sensitivity of core location for tracking the movement of lake depth associated with the benthic:planktonic (B : P) diatom boundary under different climate scenarios: (a) wetter conditions, precipitation (P) > evaporation (E); and (b) more arid conditions, P < E. The solid lines indicate the present-day B : P boundary, the dotted lines indicate the direction of movement of the B : P boundary. Under climate scenario (a) as lake level rises, the B : P boundary moves toward shore. Under climate scenario (b) as lake level declines, the B : P boundary moves toward the deeper center areas. Figure is modified from Laird et al., 2011.
ples were allowed to settle for 24 h before the acid above the sample was removed, and the sample was rinsed with distilled water. This procedure was repeated until the sample had the same pH as the distilled water (approximately eight rinses). Four successive dilutions for each sample were pipetted onto coverslips ensuring that each sample was well mixed.
Samples on the coverslips were air-dried overnight, then heated on a warming plate to remove any remaining moisture, and subsequently mounted with Naphrax® onto glass microscope slides. Diatoms were identified and counted along transects on the prepared slide using a Leica (DMRB model) microscope fitted with a 100x fluotar objective
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
MEDIEVAL CLIMATE ANOMALY NORTHWEST ONTARIO 5 (NA = 1.3) and using differential interference contrast optics at 1000x magnification. Approximately 400 diatom valves were enumerated per slide. Diatoms were identified down to the species level or lower, using taxonomic references outlined in Laird et al. (2011).
Statistical models Diatom-inferred (D-I) depth models were developed based on surface-sediment samples (~top 0.5 cm interval) collected along a depth gradient within each of the six small boreal lakes (Laird & Cumming, 2008; Laird et al., 2011). A series of surface-sediment samples were collected along a gentle slope ( ~15 m (Laird & Cumming, 2008, 2009), and the complexities of the potential for ELA Lake 239 coalescing with adjacent ELA Lake 240 with increasing depth. A widespread external forcing must be large enough for regional patterns to emerge, despite the potential heterogeneity and intrinsic complexities of local ecosystems and internal dynamics (Williams et al., 2011). In the Nebraska Sandhills (one of the few other regions with a paleolimnologicaldrought network), an analysis of five topographically closed lakes indicated relative coherency over the last 4000 years, particularly during the MCA with all lakes indicating lake-level decline (Schmieder et al., 2011). Our sites in the WRDB indicate some temporal variability, with an earlier onset in the west (~900 CE) vs. somewhat later in the east (~1000–1100 CE), indicating a potential time-transgressive response, although these periods nearly overlap when errors in radiocarbon are taken into account. Our data indicates the MCA penetrated at least as far as our easternmost lake, but without sites further east we cannot clearly define the eastward extent. In the Canadian and northern U.S. prairies to the west of the WRDB, there is high variability in diatominferred effective moisture during the MCA interval (Laird et al., 2003). This may be due to complex interactions between groundwater and climate in this region, as well as differences in the spatial pattern of precipitation and drought depending on the position and configuration of the jet stream and high-pressure ridges (Laird et al., 2003). Other proxy records from regions adjacent to the WRDB suggest intervals of drought within the MCA interval. In Minnesota, sand deposits in Mina Lake indicate large declines in lake level during the 1300s (St. Jacques et al., 2008), high eolian deposition occurred from ~1280 to 1410 CE in Elk Lake (Dean, 1997) and d18O from calcite indicated an arid period from ~1100 to 1400 CE in Steel Lake (Tian et al., 2006). In Manitoba, the cellulose d18O record from the southern basin of Lake Winnipeg indicated severe dry conditions between 1180 and 1230 CE, and a less-severe dry period from 1320 to 1340 CE (Buhay et al., 2009). Relative warm conditions during the MCA in comparison to the Little Ice Age (LIA) have been inferred from pollen records in the central boreal region of Canada and in Wisconsin
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
10 K . R . L A I R D e t a l . (Viau & Gajewski, 2009; Viau et al., 2012; Wahl et al., 2012). Many studies now suggest that the MCA was of global extent, with many regions indicating dry conditions, while others were wet (Seager et al., 2007; Helama et al., 2009; Trouet et al., 2009; Graham et al., 2011). These conclusions are based on proxy records around the world and climate models that suggest a re-organization of the climate system. While many of these studies point to conditions in the Pacific Ocean being in a persistent La Nin˜a mode, others also indicate that conditions in the North Atlantic (Feng et al., 2011; Oglesby et al., 2012), and Indian Ocean (Graham et al., 2011) also played a role in this re-organization. Observational and modeling studies also suggest that these same sea-surface temperature (SST) patterns play a major role in present-day droughts across central North America, including our study region (e.g., Hu et al., 2011). It is not that an entirely different set of controls accounted for the drought; rather the SST patterns favoring drought appear to have prevailed during much of the MCA, whereas they are much more sporadic at present.
Recent signal (last ~100 years) Recent studies show that anthropogenic forcing has a detectable influence on climate, being implicated in precipitation trends across large latitudinal bands (Zhang et al., 2007), as well as trends in river flow, winter air temperature and snow pack (Barnett et al., 2008). At the ELA, a significant upward trend in average annual temperature (mostly due to temperature increases in the winter) is accompanied by increasing rainfall, particularly between April to October (Parker et al., 2009). These recent increases in precipitation in our study region (Fig. 2a) may be related to anticipated increased poleward moisture transport into the Great Lakes region related to greenhouse-gas induced warming (Kutzbach et al., 2005). An increase in precipitation is certainly a factor in the most recent increases in the inferred depth of our lakes. However, warming air temperatures can also be associated with a warmer epilimnion in lakes which can lead to increased stratification (Schindler et al., 1996), conditions that would favor the small planktonic Discostella (formerly Cyclotella) taxa (Ru¨hland et al., 2008), the dominant planktonic diatoms in all of our lakes (Fig. S1, Table S3). In three of our lakes (Meekin, ELA L442, Gall), the percentages of Discostella taxa are at their highest during the last 100 years of the 2000-year records. Thus the magnitude of the inferred highs of these lakes was likely amplified, in part, by increased stratification. Nonetheless, this does not negate the fact that all lakes show a similar
increasing trend in inferred depth over the last 100 years, even though the degree of change may be a result from complex interaction of climatic influences on the physical properties of lakes. In Meekin Lake, there were also concurrent small increases (~2%) in two other planktonic taxa, Asterionella formosa and Fragilaria nanana (Laird et al., 2011), which may imply either increased nutrients in the metalimnion (Ru¨hland et al., 2010) or increased stratification (Longhi & Beisner, 2009). Although nutrients could potentially be a confounding influence on our inferred water-depth changes, we found that even in our moderate nutrient status lakes (Meekin and Gall, ~10–12 lg L 1 total phosphorus, TP) the assemblages were dominated by oligotrophic taxa (including planktonic Discostella), with only small percentages of more mesotrophic taxa, and the assemblages had a similar distribution with changes in lake depth in all other lakes studied (Kingsbury et al., 2012). Gall Lake has the highest TP levels (~12 lg L 1) of our study lakes, but the percent abundance of more mesotrophic taxa, such as A. formosa has changed little over the past 2000 years and never exceeded 10% of the assemblage (Haig, 2011), suggesting nutrient status in this lake has not changed substantially over this time period. Potential multiple stressors on lakes from anthropogenic influences again highlights the necessity of a regional network of sites to help decipher intrinsic factors from external forcings.
Application of this approach to other regions Regional networks of proxy-lake records provide a means of distinguishing climate-driven signals from more local influences. Our approach of utilizing nearshore sediments, as a sensitive method to track changes in diatom assemblages near the benthic to planktonic (B : P) boundary, led to the documentation of an arid MCA across the WRDB in northwest Ontario, providing convincing evidence on the northward spatial extent of this megadrought. The ability to develop robust diatom-inferred depth models across many lakes and a well-defined B : P boundary, makes this approach potentially applicable to regions dominated by drainage lakes. Consequently, this approach can provide a method to estimate changes in effective moisture in comparison to conditions today, in more humid regions where such data have been extremely difficult to acquire. Tracking changes in diatom assemblages near the B : P diatom boundary in lake-sediment cores provides a means of retrieving a drought signal from dilute lakes with outlets; however, this approach is most appropriate in small lakes (< 100–200 ha) with a gently sloping bottom that has sufficient near-shore sediment accumu-
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
M E D I E V A L C L I M A T E A N O M A L Y N O R T H W E S T O N T A R I O 11 lation. The location of the core near the present-day depth of the B : P diatom boundary (Fig. 3) makes this technique especially sensitive to detecting even lowintensity droughts (Laird et al., 2011), but needs to be based on a high-resolution (~1 m) calibration of diatom assemblages in surface sediments across a water-depth gradient in each study lake (Kingsbury et al., 2012). Although lower sedimentation rates are generally associated with near-shore cores, comparison across lakes is still realistic at multi-decadal to centennial scales. Given these caveats, this approach can provide a powerful means to reconstruct climate where more traditional paleolimnological methods based on a central core from drainage lakes are inadequate (Battarbee, 2000).
Future implications Empirical evidence for the penetration of the MCA drought into northwest Ontario, fills a geographical gap that climate models can strive to mimic for a better mechanistic understanding of the spatial pattern of the MCA. The models must now be able to simulate this expanded region of drought if they are to be considered useful in understanding how the MCA came to be. This is especially important for high spatial resolution (4–12 km), regional climate models. Current increasing trends in precipitation in northwest Ontario suggest that water scarcity may not be an issue in the near future. However, if the concurrent increasing trends in temperature continue, this could result in declines of overall effective moisture. If trends in effective moisture were to decrease, this could lead to the return of aridity, resulting in significant declines in hydropower generation, and other environmental and socioeconomic impacts. Expanding our knowledge of the spatial extent and severity of past megadroughts provides plausible scenarios for water-resource management and societal impacts. Past megadroughts have been implicated in the decline, abandonment, and collapse of societies. The collapse of the classic Maya in Mexico and the Tiwanaku in the Bolivian-Peruvian altiplano was coincident with prolonged drought (de Menocal, 2001). Large population declines and abandonment of Puebloan and Cahokia settlements in North America occurred during the megadroughts of the MCA (Cook et al., 2007). Recent tree-ring records in Asia suggest that drought may also have contributed to the fall of the Ming Dynasty (Cook et al., 2010a). What the future climate may hold for regions of North America due to global warming is unclear, but a return to the aridity of the MCA would have significant ecological, environmental, and social impacts on this region.
Acknowledgements We thank Claire Broughton, Lindsay Braeger, Thane Anderson, and Brendan Wiltse for assistance with field work. Marten Douma of the Geological Survey of Canada provided equipment and training for acquisition of lake-sediment seismic profiles. D. L. Forbes and C.T. Schafer reviewed an early draft of the paper for the GSC. Water chemistry was provided by Andrew Paterson at the Ontario Ministry of Environment, Dorset Environmental Center. We thank the three anonymous reviewers for their comments which strengthened this paper. Funding for this project comes from an NSERC Collaborative Research and Development grant in partnership with Manitoba Hydro. Conflict of interest: No authors have a conflict of interest. Author contributions K.L. and B.C. designed the study and wrote the grant proposal and paper; K.L., H.H., S.M., M.K. provided data and analyses; T.B. provided the carbon dating; M.L. provided seismic profile data; R.O. provided input of synoptic climatic interpretations. All authors contributed to the discussion, interpretation, and writing of the paper.
References Barnett TP, Pierce DW, Hidalgo HG et al. (2008) Human-induced changes in the hydrology of the Western United States. Science, 319, 1080–1083. Battarbee RW (2000) Palaeolimnological approaches to climate change, with special regard to the biological record. Quaternary Science Reviews, 19, 107–124. Bennett KD (1996) Determination of the number of zones in a biostratigraphical sequence. New Phytology, 132, 155–170. Buhay WM, Simpson S, Thorleifson H et al. (2009) A 1000 year record of dry conditions in the eastern Canadian prairies reconstructed from oxygen and carbon isotope measurements on Lake Winnipeg sediment organics. Journal of Quaternary Science, 24, 426–436. Cook ER, Seager R, Cane MA, Stahle DW (2007) North American drought: reconstructions, causes, and consequences. Earth Science Reviews, 81, 93–134. Cook ER, Anchukaitis KJ, Buckley BM, D’Arrigo RD, Jacoby GC, Wright WE (2010a) Asian monsoon failure and megadrought during the last millennium. Science, 328, 486–489. Cook ER, Seager R, Heim RR Jr., Vose RS, Herweijer C, Woodhouse C (2010b) Megadroughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science, 25, 48–61. Dean WE (1974) Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. Journal of Sedimentology Petrology, 44, 242–248. Dean WE (1997) Rates, timing, and cyclicity of Holocene eolian activity in north-central US: evidence from varved lake sediments. Geology, 25, 331–334. Dearing JA (1997) Sedimentary indicators of lake-level changes in the humid temperate zone: a critical review. Journal of Paleolimnology, 18, 1–14. Fee EJ, Hecky RE, Kasian SEM, Cruikshank DR (1996) Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian shield lakes. Limnology and Oceanography, 41, 912–920. Feng S, Hu Q, Oglesby RJ (2011) Influence of Atlantic sea surface temperature on persistent drought in North America. Climate Dynamics, 37, 569–586. Findlay DL, Kasian SEM, Stainton MP, Beaty K, Lyng M (2001) Climatic influences on algal populations of boreal forest lakes in the Experimental Lakes Area. Limnology and Oceanography, 46, 1784–1793. Fritz SC (2008) Deciphering climatic history from lake sediments. Journal of Paleolimnology, 39, 5–16. Glew JR (1989) A new trigger mechanism for sediment samplers. Journal of Paleolimnology, 2, 241–243. Glew JR, Smol JP, Last WM (2001) Sediment core collection and extrusion. In Tracking Environmental Change Using Lake Sediments: Volume 1. Basin Analysis, Coring, and Chronological Techniques (eds. Last WM, Smol JP), pp. 73–105. Kluwer Academic Publishers, Dordrecht. Graham NE, Ammann CM, Fleitmann D, Cobb KM, Luterbacher J (2011) Support for global climate reorganization during the ‘Medieval Climate Anomaly’. Climate Dynamics, 37, 1217–1245. Haig H (2011) Diatom-inferred Changes in Effective Moisture from Gall Lake, Northwestern Ontario, Over the Past Two Millennia. M.Sc. thesis, Queen’s University, Kingston, Ontario.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
12 K . R . L A I R D e t a l . Heino J, Virkkala R, Toivonen H (2009) Climate change and freshwater biodiversity: detected patterns, future trends and adaptations in northern regions. Biological
Experimental Lakes Area (Ontario, Canada). Canandian Journal of Fisheries and Aquatic Sciences, 66, 1848–1863.
Reviews, 84, 39–54. Helama S, Merila¨inen J, Tuomenvirta H (2009) Multicentennial megadrought in northern Europe coincided with a global El nin˜o -southern oscillation drought pattern during the Medieval Climate Anomaly. Geology, 37, 175–178. Hu Q, Feng S, Oglesby RJ (2011) Variations in north American summer precipitation driven by the Atlantic multidecadal oscillation. Journal of Climate, 24, 5555–5570. Juggins S (2003) C2 Software for Ecological and Palaeoecological Data Analysis and Visuali-
Reimer PJ, Baillie MGL, Bard E et al. (2009) INTCAL09 and MARINE09 radiocarbon age calibration curves, 0-50 000 years cal BP. Radiocarbon, 51, 1111–1150. Romm J (2011) The next dust bowl. Nature, 478, 450–451. Ruckstuhl KE, Johnson EA, Myianishi K (2008) The boreal forest and global change. Philosophical Transactions of the Royal Society B, 363(1501), 2245–2249. Ru¨hland K, Paterson AM, Smol JP (2008) Hemispheric-scale patterns of climaterelated shifts in planktonic diatoms from North American and European lakes.
zation User Guide Version 1.3. University of Newcastle, Newcastle, UK. Karlsson J, Bystro P, Ask J, Persson L, Jansson M (2009) Light limitation of nutrientpoor lake ecosystems. Nature, 460, 506–510. Keller W (2007) Implications of climate warming for Boreal shield lakes: a review and synthesis. Environmental Reviews, 15, 99–112. Kingsbury MV, Laird KR, Cumming BF (2012) Consistent patterns in diatom
Global Change Biology, 14, 2740–2754. Ru¨hland KM, Paterson AM, Hargan K, Jenkin A, Clark BJ, Smol JP (2010) Reorganization of algal communities in the lake of the Woods (Ontario, Canada) in response to turn-of-the-century damming and recent warming. Limnology and Oceanography, 55, 2433–2451. Schelske CL, Peplow A, Brenner M, Spencer CN (1994) Low-background gamma
assemblages and diversity measures across water-depth gradients from eight Boreal lakes from northwestern Ontario (Canada). Freshwater Biology, 57, 1151– 1165. Kutzbach JE, Williams JW, Vavrus SJ (2005) Simulated 21st century changes in regional water balance of the Great lakes region and links to changes in global temperature and poleward moisture transport. Geophysical Research Letters, 32, L17707, doi:10.1029/2005GL023506.
counting: applications for 210Pb dating of sediments. Journal of Paleolimnology, 10, 115–128. Schindler DW, Donahue WF (2006) An impending water crisis in Canada’s western prairie provinces. Proceedings of the National Academy of Sciences, 103, 7210–7216. Schindler DW, Lee PG (2010) Comprehensive conservation planning to protect biodiversity and ecosystem services in Canadian boreal regions under a warming climate and increasing exploitation. Biological Conservation, 143, 1571–1586.
Laird KR, Cumming BF (2008) Reconstruction of Holocene lake level from diatoms, chrysophytes and organic matter in a drainage lake from the Experimental Lakes Area (northwestern Ontario, Canada). Quaternary Research, 69, 292–305. Laird KR, Cumming BF (2009) Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area, northwestern Ontario, Canada. Journal of Paleolimnology, 42, 65–80.
Schindler DW, Bayley SE, Parker BR et al. (1996) The effects of climatic warming on the properties of boreal lakes and streams at the Experimental Lakes Area, northwestern Ontario. Limnology and Oceanography, 41, 1004–1017. Schmieder J, Fritz SC, Swinehart JB et al. (2011) A regional-scale climate reconstruction of the last 4000 years from lakes in the Nebraska sand hills, USA. Quaternary Science Reviews, 30, 1797–1812.
Laird KR, Cumming BF, Wunsam S, Rusak JA, Oglesby RJ, Fritz SC, Leavitt PR (2003) Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia. Proceedings National Academy of Science, 100, 2483–2488. Laird KR, Kingsbury MV, Cumming BF (2010) Assessment of diatom species habitats, species diversity and depth-inference models across multiple transects in Worth lake, northwestern Ontario, Canada. Journal of Paleolimnology, 44, 1009–
Seager R, Graham N, Herweijer C, Gorodn AL, Kushnir Y, Cook E (2007) Blueprints for medieval hydroclimate. Quaternary Science Reviews, 26, 2322–2336. Snucins E, Gunn J (2000) Interannual variation in the thermal structure of clear and colored lakes. Limnology and Oceanography, 45, 1639–1646. St. George S (2007) Streamflow in the Winnipeg river basin, Canada: trends, extremes and climate linkages. Journal of Hydrology, 332, 396–411. St. George S, Meko DM, Evans MN (2008) Regional tree growth and inferred summer
1024. Laird KR, Kingsbury MV, Lewis CFM, Cumming BF (2011) Diatom-inferred depth models in 8 Canadian Boreal lakes: inferred changes in the benthic:planktonic depth boundary and implications for assessment of past droughts. Quaternary Science Reviews, 30, 1201–1217. Longhi ML, Beisner BE (2009) Environmental factors controlling the vertical distribu-
climate in the Winnipeg river basin, Canada, since AD 1783. Quaternary Research, 70, 158–172. St. Jacques JM, Cumming BF, Smol JP (2008) A 900-year pollen-inferred temperature and effective moisture record from varved lake mina, west-central Minnesota, USA. Quaternary Science Reviews, 27, 781–796. Stuiver M, Reimer PJ (1993) Extended 14C database and revised CALIB radiocarbon
tion of phytoplankton in lakes. Journal of Plankton Research, 31, 1195–1207. Ma S (2011) Climate change and water availability over the last two millennia in Little Raleigh lake, northwestern Ontario. M.Sc. thesis, Queen’s University, Kingston, Ontario. Mekis E´, Vincent LA (2011) An overview of the second generation adjusted daily precipitation dataset for trend analysis in Canada. Atmosphere-Ocean, 49, 163–177. de Menocal PB (2001) Cultural responses to climate during the late Holocene. Science,
calibration program. Radiocarbon, 35, 315–330. Tian J, Nelson DM, Hu FS (2006) Possible linkages of late-Holocene drought in the North American midcontinent to Pacific decadal oscillation and solar activity. Geophysical Research Letters, 33, L23702, doi: 10.1029/2006GL028169 Trouet V, Esper J, Graham NE, Baker A, Scourse JD, Frank DC (2009) Persistent positive North Atlantic oscillation mode dominated the Medieval Climate Anomaly. Science, 324, 78–80.
292, 667–673. Miao X, Mason JA, Swinehart JB, Loope DB, Hanson PR, Goble RJ, Xiaodong L (2007) A 10 000 year record of dune activity, dust storms, and severe drought in the central Great plains. Geology, 35, 119–122. Moos MT, Laird KR, Cumming BF (2009) Climate-related eutrophication of a small boreal lake in northwestern Ontario: a palaeolimnological perspective. The Holo-
Viau AE, Gajewski K (2009) Reconstructing millennial-scale, regional paleoclimates of boreal Canada during the Holocene. Journal of Climate, 22, 316–330. Viau AE, Ladd M, Gajewski K (2012) The climate of North America during the past 2000 years reconstructed from pollen data. Global and Planetary Change, 84-85, 7, 5–83. Wahl ER, Diaz HF, Ohlwein C (2012) A pollen-based reconstruction of summer temperature in central North America and implications for circulation patterns during
cene, 19, 359–367. Oglesby R, Feng S, Hu Q, Rowe C (2012) The role of the Atlantic multidecadal oscillation on medieval drought in North America: Synthesizing results from proxy data and climate models. Global and Planetary Change, 84-85, 56–65. Oldfield F, Appleby PG (1985) Empirical testing of 210Pb-dating models for lake sediments. In Lake Sediments and Environmental History (eds Haworth EY, Lund JWG), pp. 93–124. University of Minnesota Press, Minneapolis.
medieval times. Global and Planetary Change, 84-85, 6, 6–74. White DB (2000) Ontario Land Cover Data. G.A.D.A.S. Section & N.R.I. Branch. Ontario Ministry of Natural Resources, Peterborough, Ontario. Williams JW, Blois JL, Shuman BN (2011) Extrinsic and intrinsic forcing of abrupt ecological change: case studies from the late Quaternary. Journal of Ecology, 99, 664–677. Williamson CE, Saros JE, Schindler DW (2009) Sentinels of change: lakes and reservoirs provide key insights into the effects and mechanisms of climate change. Sci-
Ontario Ministry of Natural Resources (2000) Forest Resources Inventory (FRI) Database, Ontario Ministry of Natural Resources, Queen’s University, Kingston ON. Pace ML, Cole JJ (2002) Synchronous variation of dissolved organic carbon and color in lakes. Limnology and Oceanography, 47, 333–342. Parker BR, Schindler DW, Beaty KG, Stainton MP, Kasian SEM (2009) Long-term changes in climate, streamflow, nutrient budgets for first-order catchments at the
ence, 323, 887–888. Winter TC, Woo M (1990) Hydrology of lakes and wetlands. In Surface Water Hydrology (eds Wolman MG, Riggs HC), pp. 158–188. Geological Society of America, Boulder, CO. Zhang X, Zwiers FW, Hegerl GC et al. (2007) Detection of human influence on twentieth-century precipitation trends. Nature, 448, 461–465.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
M E D I E V A L C L I M A T E A N O M A L Y N O R T H W E S T O N T A R I O 13 Supporting Information Additional Supporting Information may be found in the online version of this article: Figure S1. Percentage of planktonic diatoms in the cores retrieved from the six study lakes. Figure S2. Percentage of benthic diatoms in the cores retrieved from the six study lakes. Table S1. Summary of some physical and chemical characteristics for each of the study lakes Table S2. Results of AMS radiocarbon dates processed at the Center for AMS at Lawrence Livermore National Laboratory Table S3. List of dominant and sub-dominant planktonic diatom taxa for each of the six lakes Table S4. List of dominant and sub-dominant benthic diatom taxa for each of the six lakes
Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.
© 2012 Blackwell Publishing Ltd, Global Change Biology, doi: 10.1111/j.1365-2486.2012.02740.x
